1. Field of the Invention
The present invention relates to coding/decoding of an interlaced video, and more particularly to an apparatus and method of adaptively coding/decoding interlaced shape information, for coding and decoding shape information in units of frame type or field type based upon characteristics of the shape information when coding interlaced video, thereby having a high compression ratio.
2. Description of Related Art
For the future video/audio technology and system, development is in progress to make it realize to support novel video/audio applications which existing known standards (e.g., H.263 and H.261 by ITU-T and MPEG1 and MPEG-2 by ISO/IEC) cannot support. An object-based interactive functionality and an object-based manipulation are representative examples of new functionality. To offer novel and various functionality, transmission of shape information is required. The shape information is for dividing a picture into an object region and a non-object region (background). This shape information allows for transmitter and receiver to perform signal processing focusing on the object region instead of the whole picture and offers the novel functionalities. Binary shape information has a form of a binary mask where pixels have different values according to whether the pixel is for the object or non-object. For example, the values of pixels for the object and non-object(background) are 0 and 1, respectively. It should be noted that these values are not limited to a specific value. A video coding method using such shape information is called object-based video coding.
The shape information has a great amount of data, so it is deemed to be important how to effectively compress the shape information. This aims at improvement of efficiency in compression coding of the shape information.
FIG. 1 is a block diagram of a general object-based video coder.
Signals of a picture consists of shape information and texture information and two types of information are respectively inputted to a shape coding unit 11 and motion estimation unit 12
The shape coding unit 11 performs loss coding or lossless coding with respect to the shape information in each picture. Reconstructed shape information is inputted to both motion compensation unit 13 and texture coding unit 17. Both motion compensation unit 13 and texture coding unit 17 operate based upon an object. A shape bitstream, which is the other output of the shape coding unit 11, is inputted to a multiplexer 18.
The motion estimation unit 12 estimates motion information of current picture's texture information using input texture information of the current picture and texture information of a previous picture which is stored in a previous reconstructed frame memory 14. Estimated motion information is inputted to the motion compensation unit 13 while a motion information is encoded and its bitstream is inputted to the multiplexer 18. The motion compensation unit 13 performs motion compensated prediction using the motion information obtained through the motion estimation unit 12 and the previous reconstructed picture in the previous reconstructed frame memory 14.
The texture coding unit 17 encodes a prediction error which is a difference between an input texture information and motion compensated texture information, and is obtained at a subtracter 15. A texture bitstream which is produced through the coding is inputted to the multiplexer 18 and an error signal of reconstructed texture information is inputted to an adder 16. The previous reconstructed frame memory 14 stores a previous reconstructed picture signal produced by the adder 16 which adds the error signal to a motion compensated prediction signal
Digital video may be classified into progressive video and interlaced video according to frame constructing methods. For the progressive video, a frame is constructed in such a manner that lines subsequently progresses from the top to the bottom. For the interlaced video, a frame is constructed in such a manner that two fields are separately constructed and then lines of two fields are interlaced for constructing a frame. A height (the number of lines) of a field is a half of the height of a frame. This is illustrated in FIG. 2. FIG. 2a shows a frame in the progressive video and FIG. 2b shows two fields--a top field and a bottom field--and a frame in the interlaced video. In FIG. 2, the lines of the top and bottom fields are denoted as solid arrows and dashed arrows, respectively, and the lines of each field are interlaced (the solid arrows are interlaced with the dashed arrows) to construct an interlaced frame.
When the top and bottom fields are constructed, as shown in FIG. 2b, there is a time lag between the two fields and the top field precedes the bottom field. In other cases, the bottom field may precede the top field. Because of the time lag between the top field and the bottom field, signal characteristics of two adjacent lines in the interlaced frame can be different. Particularly in case of a picture having a lot of motion, this feature is prominent. When applying video coding tools developed in accordance with properties of the progressive video, such as motion estimation, motion compensation, and discrete cosine transform (DCT), coding efficiency may be reduced in the case of interlaced video coding. Technology such as field-based motion estimation and compensation and adaptive frame/field DCF has been developed to prevent this problem. Such technology is disclosed in the standard MPEG-2 and has been frequently applied to actual application products such as digital TV. However, the technologies for coding/decoding the interlaced shape information has not been presented as yet because the adoption of the shape information was investigated most recently. Therefore, this technology has a very important role in the (future) multimedia applications including interlaced video coding.
FIGS. 3a and 3b show interlaced shape information where an object has no motion or a little motion between two fields. As shown in FIG. 3a, correlation between lines in a frame is higher compared with that in each field, so it is better to code the shape information in the unit of frames in this case.
FIGS. 4a and 4b show interlaced shape information where an object has much motion between two fields. As shown in FIG. 4b where the lines are grouped into each field, variation between neighboring lines in each field is little and correlation between lines is high in the same field. However, as shown in FIG. 4a, considering in units of a frame type, the variation between neighboring lines is larger and the correlation between lines is lower. Therefore, coding efficiency is reduced when coding the shape information in the unit of frames. To overcome this problem, the present invention provides a method of coding the shape information having a lot of motion in units of a field type.
Furthermore, it is best to adaptively select one of a field type coding and frame type coding rather than to use only one type coding with respect to the interlace shape information.